Sustained-release preparations for the adjustment of blood concentrations of drugs are highly useful in terms of separation between the main pharmacological effect and adverse reaction, improvement in compliance (e.g., the number of doses reduced by improvement in prolonged efficacy), medical economy, etc. In this regard, some techniques have been reported for sustained-release preparations. Meanwhile, since compounds exhibiting the main pharmacological effect have diverse chemical properties, some sustained release techniques, albeit still insufficient, adaptable to the diverse chemical properties of these compounds have been reported (see e.g., Patent Documents 1 and 2).
The properties of a drug itself can be classified broadly into neutral, acidic, and basic properties. Among others, solubility (degree of solubility) in water differs greatly between compounds. Low water-soluble compounds have many disadvantages in the design of preparations to improve dissolution properties. Acidic drugs refer to acidic compounds that are acidic in the free form (whose acidic group does not constitute a salt such as an alkali- or amine-adduct salt). Acidic drugs are disadvantageously low soluble in acidic solutions, for example, in the upper gastrointestinal tract such as the stomach. A salt (alkali- or amine-adduct salt) of an acidic compound disadvantageously becomes a low soluble free acid in an acidic solution. Alternatively, basic drugs refer to basic compounds that are basic in the free form (whose basic group does not constitute a salt such as an acid-adduct salt) and are known to exhibit favorable solubility in strongly acidic aqueous solutions, but exhibit reduced solubility in neutral aqueous solutions such as a neutral buffer. Specifically, basic drugs, when orally administered, exhibit favorable solubility in the stomach, which is acidic. Their solubility, however, is greatly reduced in the lower gastrointestinal tract such as the large intestine, which is neutral with little water, probably leading to a reduced absorption rate of the drug.
For example, a challenge for the design of sustained-release preparations for oral administration containing a basic drug is dose dumping of the drug when the preparation collapses due to mechanical stress resulting from the presence of food in the acidic environment of the upper gastrointestinal tract exhibiting high water-solubility, gastrointestinal motility, and so on. Furthermore, preparation strength may be enhanced by, for example, an increased amount of a sustained-release agent in order to avoid dose dumping of the drug. In such a case, still, the challenge for a sustained-release preparation containing a basic drug whose water solubility is reduced in the neutral region is to improve the dissolution properties of the preparation in the lower gastrointestinal tract and maintain drug absorption. No previous technique for sustained-release preparations containing a basic drug can simultaneously achieve, at satisfactory levels, avoidance of dose dumping of the drug in an acidic environment such as the upper gastrointestinal tract and prolonged dissolution in the lower gastrointestinal tract, which is a neutral environment.